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Re: AI-GEOSTATS: Choosing Lag Distance and Angular Tolerance

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  • Isobel Clark
    Think of it as focussing a camera. You are looking for the clearest picture. There will be a certain lag (in each direction) at which the calculation will
    Message 1 of 5 , May 15 6:05 AM
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      Think of it as focussing a camera. You are looking for
      the clearest picture. There will be a certain lag (in
      each direction) at which the calculation will balance
      between detail on distance, detail on angles and
      number of pairs.

      In general:

      # the more anisotropic the structure, the smaller the
      tolerance on angle should be

      # a natural interval can be found from the 'nearest
      neighbour' distances between sample locations

      # you won't hit it on the first shot!

      Isobel Clark
      http://uk.geocities.com/drisobelclark

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    • Andrew Mullens
      I have a question relating to this question, certainly not to question the previous writer, it just seems like a good time to bring it up. Will variograming
      Message 2 of 5 , May 15 8:47 AM
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        I have a question relating to this question, certainly not to question the
        previous writer, it just seems like a good time to bring it up. Will
        variograming and other such techniques work for the data the previous writer
        described, e.g samples aren't at points, but areas (and areas that might
        have very little to do with the question). If they did use points in the
        calculations where would the points be placed, at the center of the county,
        at the major population center, at some arbitrary point (e.g most northerly
        point).

        I may be miss reading the description, perhaps the sample are point samples,
        but were taken with one sample in each county.

        Obviously the point samples are never really point sample, they must be
        taken over some area, approximating a point, but does this design seem to
        push the boundaries on that assumption.

        Andrew
        ----- Original Message -----
        From: "Yadollah Waghei" <waghei@...>
        To: <ai-geostats@...>
        Cc: <waghei_y.MED.TMU@...>;
        <mohsen_m@...>; <waghei@...>;
        <mohsen_m@...>; <kazem_an@...>; <Faghihzadeh@...>
        Sent: Tuesday, May 15, 2001 7:33 AM
        Subject: AI-GEOSTATS: Choosing Lag Distance and Angular Tolerance


        > Hello dears
        > I have a spatial data set contaning n=262 observarion (The variable of
        interest is Rate of Tuberculosis in 262 counties of Iran). I want to fit
        some models to Directional semi-variograms,and then build anisotropic
        semi-variogram.
        > Then questions are
        > - Is there any rule for choosing Lag Distance and Angular Tolerance?
        > -Also,how we can balance between Lag Distance and Angular Tolerance?
        > -Do you agree that both must be very small,as possible?(Such that number
        of pairs in each lag>20, for example)
        >
        > Thank you
        > Yadollah Waghei
        > Dep.of Biostatistics
        > Tarbiat Modarres Univ.(Tehran)Po.Box: 14115-111
        > Tel:8011001-3872 Fax:8007989
        >
        ___________________________________________________________________________
        > Visit http://www.visto.com/info, your free web-based communications
        center.
        > Visto.com. Life on the Dot.
        >
        >
        > --
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        >


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      • Isobel Clark
        Andrew You can apply standard geostatistics if the measurements are the average (or some similar feature) over an area. It makes interpeting the
        Message 3 of 5 , May 15 9:52 AM
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          Andrew

          You can apply 'standard' geostatistics if the
          measurements are the 'average' (or some similar
          feature) over an area.

          It makes interpeting the semi-variogram extremely
          tricky if you combine many different sizes of sample,
          but common sense is the main thing here. The trick is
          to derive a point semi-variogram model from which any
          size can be derived (see Chapter 3, Practical
          Geostatistics 1979, downloadable free from Web,
          http://uk.geocities.com/drisobelclark/practica.html)

          Kriging is modified to reflect that the samples are
          averages, mainly by changing the diagonal elements in
          the equations so that they are non-zero. I don't know
          any software package (off hand ) that does this,
          though.

          Isobel Clark



          --- Andrew Mullens <andrew_mullens@...>
          wrote: > I have a question relating to this question,
          > certainly not to question the
          > previous writer, it just seems like a good time to
          > bring it up. Will
          > variograming and other such techniques work for the
          > data the previous writer
          > described, e.g samples aren't at points, but areas
          > (and areas that might
          > have very little to do with the question). If they
          > did use points in the
          > calculations where would the points be placed, at
          > the center of the county,
          > at the major population center, at some arbitrary
          > point (e.g most northerly
          > point).
          >
          > I may be miss reading the description, perhaps the
          > sample are point samples,
          > but were taken with one sample in each county.
          >
          > Obviously the point samples are never really point
          > sample, they must be
          > taken over some area, approximating a point, but
          > does this design seem to
          > push the boundaries on that assumption.
          >
          > Andrew
          > ----- Original Message -----
          > From: "Yadollah Waghei" <waghei@...>
          > To: <ai-geostats@...>
          > Cc: <waghei_y.MED.TMU@...>;
          > <mohsen_m@...>; <waghei@...>;
          > <mohsen_m@...>; <kazem_an@...>;
          > <Faghihzadeh@...>
          > Sent: Tuesday, May 15, 2001 7:33 AM
          > Subject: AI-GEOSTATS: Choosing Lag Distance and
          > Angular Tolerance
          >
          >
          > > Hello dears
          > > I have a spatial data set contaning n=262
          > observarion (The variable of
          > interest is Rate of Tuberculosis in 262 counties of
          > Iran). I want to fit
          > some models to Directional semi-variograms,and then
          > build anisotropic
          > semi-variogram.
          > > Then questions are
          > > - Is there any rule for choosing Lag Distance and
          > Angular Tolerance?
          > > -Also,how we can balance between Lag Distance and
          > Angular Tolerance?
          > > -Do you agree that both must be very small,as
          > possible?(Such that number
          > of pairs in each lag>20, for example)
          > >
          > > Thank you
          > > Yadollah Waghei
          > > Dep.of Biostatistics
          > > Tarbiat Modarres Univ.(Tehran)Po.Box: 14115-111
          > > Tel:8011001-3872 Fax:8007989
          > >
          >
          ___________________________________________________________________________
          > > Visit http://www.visto.com/info, your free
          > web-based communications
          > center.
          > > Visto.com. Life on the Dot.
          > >
          > >
          > > --
          > > * To post a message to the list, send it to
          > ai-geostats@...
          > > * As a general service to the users, please
          > remember to post a summary of
          > any useful responses to your questions.
          > > * To unsubscribe, send an email to
          > majordomo@... with no subject and
          > "unsubscribe ai-geostats" followed by "end" on the
          > next line in the message
          > body. DO NOT SEND Subscribe/Unsubscribe requests to
          > the list
          > > * Support to the list is provided at
          > http://www.ai-geostats.org
          > >
          >
          >
          > --
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          > to post a summary of any useful responses to your
          > questions.
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        • Isobel Clark
          ... wrote: ... This is something I learned about in quantitative ecology in the 1960 s ;-) You take each sample in turn and find the sample closest to it.
          Message 4 of 5 , May 15 1:28 PM
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            --- Lucie Vallée <Lucie_Vallee@...>
            wrote: >
            >
            > Could you explain what you mean by "nearest
            > neighbour distances between samples locations"?
            This is something I learned about in quantitative
            ecology in the 1960's ;-)

            You take each sample in turn and find the sample
            closest to it. Note the distance. A histogram of the
            distances will give you a sort of natural inter-sample
            distance for use in the semi-variogram calculation.

            The histogram can also be used to check whether
            sampling is randon, uniform, clustered etc.

            I don't have a full reference but it should be easy to
            find: Pielou, Cressie or any other book with Spatial
            Statistics in the title.

            Isobel Clark


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